1. Field of the Invention
The present invention relates to a method of processing controlled variables in an engine control system, and more particularly to a method of processing controlled variables in an electronic engine control system for controlling an engine based on various data items produced during operation of the engine.
2. Description of the Prior Art
There are known various electronic engine control systems for controlling engine operation, employing a microcomputer for processing various controlled variables such as the amount of fuel to be injected, ignition spark timing, and the amount of exhaust gas to be sent back through the engine in EGR (Exhaust Gas Recirculation) based on various data items representative of engine operating conditions such as the amount of intake air, and the rotational speed of the engine, etc. For example, U.S. Pat. No. 3,969,614 issued to David F. Moyer et al on July 13, 1976 discloses an engine control system that is programmed for simultaneously processing various controlled variables on a real-time basis. Since many controlled variables must be processed at the same time in the disclosed engine control system, the microcomputer in the engine control system is required to have a processing capacity large enough to execute many arithmetic operations for meeting the above requirement. Therefore, the design of the circuit arrangement of the engine control system is complex, and the engine control system is highly costly to manufacture.
U.S. Pat. No. 4,163,282 issued to Yamada et al on July 31, 1979 discloses a method of processing controlled variables in an engine control system in order to solve the problems of the above conventional engine control system. In this prior art, control variables , requiring high-accuracy control, such as the amount of fuel to be injected and ignition spark timing, and the amount of exhaust gas to be sent back in EGR, are given respective priorities dependent on the frequency at which they are processed, and are processed according to the given priority sequence. More specifically, the amount of fuel to be injected and ignition spark timing, which are given the first priority, are processed when an interrupt is caused by a crank pulse that is generated by a crank angle sensor each time the crankshaft rotates through a predetermined angle. After the first priority task is finished, the amount of exhaust gas in EGR, or the second priority task, is processed by an interrupt that occurs in response to a timer pulse produced from a timer circuit. When an interrupt is requested by a crank pulse while the amount of exhaust gas in EGR is being processed, control is transferred from the second priority task to the first priority task so that the amount of fuel to be injected and ignition spark timing are processed. After completion of the first priority task, control returns to the processing of the amount of exhaust gas in EGR.
The above processing method is however disadvantageous for the following reason: In a higher engine rotation range, crank pulses are produced at smaller intervals. Therefore, the time required for processing the amount of fuel to be injected, for example, upon a crank- pulse-initiated interrupt request occupies a large proportion in the entire operation time of the engine control system. The processing operation for the amount of fuel to be injected thus tends to be limited.